1. Field of the Invention
This invention relates generally to electrical engineering, and more particularly to microswitches.
2. Description of the Prior Art
The process of contact switching in limit switches is normally induced by the action of a movable cam on the drive element of the switch. At low speeds of operation of the machine cam member and at a speed of contact switching dependent on the speed of travel of the cam, as is the case with direct action switches, the interval between the initial engagement of contacts under load and the point when a suffient contact pressure is developed between the contacts is relatively lengthy. The prolonged closed position of the contacts under electric load accompanied by insufficient contact pressure, as well as slow switching action of the contacts may cause numerous emergencies, such as fusing, burning and welding of contacts.
In order to obviate the aforedescribed contact damage at low speeds of travel of the movable cam, use is generally made of microswitches provided with means for accelerating the speed of contact switching.
There is known a microswitch (cf., e.g. U.S. Pat. No. 2,791,656) comprising a base, fixed contacts secured on the base, and movable contacts connectable with the fixed contacts. Connected to the movable contacts is a drive member, this connection being executed by means of a mechanism for switching the movable contacts in the form of a resilient strip. Immediately in the central portion of the resilient strip there are arranged the movable contacts. At the portions between the attachment points of the movable contacts and their outer ends connected to the drive member the resilient strip has two resilient elements disposed symmetrically relative to the axis of the drive member at an angle to each other for swinging angularly relative to each other. This microswitch is simple to construct and has relatively small dimensions.
Operation of this microswitch is characterized in that during the movement of the drive member the ends of the resilient elements of the mechanism for switching the movable contacts connected therewith tend to be displaced. Therewith, the resilient elements change their position relative to the movable contacts, whereas the contact pressure varies depending on the speed of travel of the drive member from its nominal value to the minimum. Upon the drive member reaching the movable contacts the resilient elements assume substantially horizontal position, while the contact pressure drops practically to zero. A slow or reverse movement of the drive member may result in that the contact pressure may be close to zero for a considerable length of time, which in turn may cause failure of the microswitch.
When operation of the aforedescribed microswitch is accompanied by vibrations or impacts, even negligeable displacements of the drive member and movable contacts upset the position of the spring elements relative to the movable contacts and lead, accordingly, to minimized contact pressure.
When the drive member assumes a position close to contact making or breaking, the fully deformed resilient strip is almost horizontal, whereas its central portion is pressed by the movable contacts to the fixed contacts with a minimal force to result in unstable position of the resilient strip. Even small vibrations or impact forces acting on the microswitch in this position of the drive member may cause accidental switching of the movable contacts when the drive member is short of the position for contact making or breaking, or triggering. For this reason, the above microswitch fails to provide a reliable triggering accuracy (repeatability of triggering points) when the drive member moves slowly or reversed, or when the microswitch is subjected to vibrations and shocks.
Such a low accuracy of the microswitch and insufficiently reliable operation thereof during slow or reversed movement of the drive member accompanied by vibrations and impacts exerted on the microswitch limits the field of its application.
There is also known a microswitch (cf., U.S. Pat. No. 2,125,432) comprising a base, fixed contacts secured on the base, and movable contacts connectable with the fixed contacts. A drive member is linked with the movable contacts by way of a mechanism for switching the movable contacts defined by two springs disposed symmetrically relative to the axis of the drive member. Outer ends of the springs are connected to the drive member for angular swing relative thereto. Inner ends of the springs are connected to the contact holder on which the movable contacts are secured. The positioning of the movable contacts on the contact holder outside the mechanism for switching the movable contacts makes it possible to operate the microswitch under higher current loads.
However, this modified form of a microswitch also fails to assure reliable operation when the drive member thereof is moved slowly or reversed, or when the microswitch is subjected to vibrations and impacts due to that the position of the springs relative to the fact the movable contacts tends to change before the triggering point.
There is also widely known a microswitch (cf., e.g., U.S. Pat. No. 3,764,761) comprising a base, fixed contacts secured on the base, and movable contacts connectable with the fixed contacts. Connected to the movable contacts is a drive member, this drive member being thus connected by means of a mechanism for switching the movable contacts having at least two assemblies disposed symmetrically relative to the axis of the drive member. Each of the assemblies is comprised of a lever and a spring.
Outer ends of the levers and springs are interconnected to be capable of swinging angularly relative to each other. The outer ends of the levers are thrust against stop elements arranged on the base. Inner ends of the springs are connected to the drive member for swinging angularly relative thereto. Inner ends of the levers carrying the movable contacts are interconnected for angular swing relative to the axis of the drive member. The distance between the points of contact of the outer ends of the levers with the stop elements is greater than the distance between the attachment points of the movable contacts on the inner ends of the levers.
In the microswitch described heretofore the movement of the drive member to the point of triggering practically does not cause a change in the relative angular position of the levers on which the movable contacts are secured. Therefore, the contact pressure is not dependent on the speed of movement of the drive member or its position.
Further, small displacements of the drive member or movable contacts under the action of the forces of vibration or impact result in an increase in the contact pressure, whereby the above known microswitch is sufficiently resistant to vibrations and impacts.
However, this microswitch is difficult to construct, is bulky and has too many parts. The length of such a microswitch is determined by the length of two levers and the mechanism for switching the movable contacts interposed between the levers, whereas the width of the microswitch is determined by the width of the contact levers and two dimensions of the flat lateral springs of significant width.